First, I don't think they are traveling time. They are relaying thoughts to and from the future and past.

There's a theory that the aliens already understood human language. We just thought they didn't because we didn't understand theirs. So we started with the basics and built upon that because of our notion that they didn't understand, when really we didn't understand and they probably were just frustrated by that fact and waited for us to catch up.

So they could have and probably did relay their ideas to their future selves but it wasn't their understanding of us that needed to catch up it was our understanding of them.

She was born after the heptapods came so it's no surprise she was making sculptures of them. The sculptures are only a clue to the viewer that our heroine is actually having a "flash forward" and not a flashback. There's nothing to indicate Hanna has any kind of gift.

If you go to walmart.com om your mobile and search for your item, it will tell you the aisle the item is in.

Yeah... If it's where it's supposed to be.

I've ordered an item from them before and sat there for twenty minutes while they tried to find it. Left and they said they'd call me when they found it. They called me and said they couldn't so I just got a refund.

They went back to our time because they knew they had to (seeing time non-linearly) to help humans survive so that in the future, we help cephalopods survive, and thus evolve into the heptapods.

I don't think this is likely. If they're here visiting now, they didn't need our help to survive as cephalopods or to evolve. They may need our help to survive in the future though.

One idea is that a sudden event eradicated them all in an instant so they have no idea what actually happened. All they would know is that there are suddenly no "memories" after a certain instant in time in the future.

Humans think causally, before and after, as opposed to the heptapods. Perhaps they need our help in predicting this event. As you stated, maybe they came back to ensure we would survive as a species so we can be there to help predict this event as we otherwise would have been extinct.

No but nearly all exterior doors do have three hinges. Installing doors for years, I can't recall ever seeing an exterior door with only two hinges. Interior doors, sure, but your entry doors have three for safety.

Regardless this is a horrible idea especially if it's only one inch screws holding those hinges to the door and frame.